Earth's Rotation

Earth's Rotation and the Apparent
Daily Motion of the Sky

The earth rotates about an imaginary line that passes through the North
and South Poles of the planet. This line is called the axis of
rotation. Earth rotates about this axis once each day (approximately
24 hours). Although you most likely already knew that fact, there
is a slight complication most people are not aware of.

More
specifically, our rotation period (the time elapsed for one rotation)
with respect
to the stars is called a sidereal day. A sidereal day is
24 sidereal hours, or 23 hours and 56 minutes on a normal clock. Our
clock time is based on the earth's rotation with respect to the sun from
solar noon to solar noon. This is a solar day, and it is divided
into 24 hours. Because Earth travels about 1 / 365 of the way around
the sun during one day, there is a small difference between solar
time and sidereal
time.

The earth takes about 1/365 of a day or about 4 minutes
more to get into the </FONT>same
position with respect to the sun after it reaches the same position with respect
to the stars. We use sun-based time because it is more important to most of us
whether the Sun is up than whether a given star is up. Those who care which star
is up (like astronomers) may also use sidereal time.

1)The sidereal day is shorter
than the solar day because the earth rotates and revolves
in the same sense - counterclockwise in the diagram above. Venus
rotates
in the opposite
direction to its revolution - is its sidereal day longer or shorter
than its solar day?

Note:
Astronomers and other scientists sometimes use ordinary words
but with different or more limited meaning attached. The words "rotate" and "revolve" are
examples of this: To an astronomer, "rotate" = spin
around an axis that runs through you, and "revolve" = orbit
about another object. One time around = one revolution. In
the 1500's Copernicus published a radical theory of the solar system
in which he proposed that the planets revolve about the sun. His
book "on the revolutions of the planets" changed how
we view our universe. From its title we get the word "revolution" meaning
"action giving rise to radical change in society".

Not
quite sure why the solar day lasts 4 minutes longer than the sidereal
day? Click on Brisban's icon for
NO FRILLS HELP!

So
which way does the world turn? One way to visualize how the
earth turns is to hold out your right fist with your thumb extended
and pointing straight up
(the traditional hitchhiker fist). If you visualize that
your thumb points north, then your fingers are curling in the direction
of Earth's rotation. Because of this easy set up, we say that
the earth rotates in a "right-handed" manner (because you are using
your right hand as a model).

You
can take this model a step further by visualizing that the base of
your thumb is where Europe and Africa are located (Europe is
above Africa of course when North is up). The Atlantic Ocean
would be on your fingers and the flat part of your fist by your knuckles
are the Americas. The back of your hand is the Pacific Ocean,
which leaves Asia to rest where your wrist is. If you keep
this simple model in mind, you will always have an Earth-globe to
check directions by. This is not so different from counting
using your fingers.

For an observer at a fixed position
on Earth, the rotation of the earth makes it appear as if the sky
is revolving around the earth. In other words, if you are standing
for long enough in a field at night, it looks like the sky is moving,
not you. This motion is called "apparent diurnal motion." "Diurnal" means
having to do with a day, in the sense of a 24-hour period.

Please
note that the word "day" has two popular meanings, which
can lead to confusion in astronomical contexts. Day can mean
when it is light out, as in "day or night." Or, it can
indicate a 24-hour period. Here we only use the second meaning. You
might want to be careful to do the same.

In
the diagram to the right we show the earth as seen from space with
Ames, Iowa, the home of Iowa State, at the top. This means
that a person in Ames would be standing with his or her head towards
the
top of the diagram. This picture may seem confusing at first
because we like to think of north as always being up, but remember
that up and down are arbitrary designations in space.

People in Australia, in the Southern Hemisphere of our
planet, certainly do not consider themselves as being upside down.

The
earth rotates around the North – South (N-S) line, so that if we
watched it rotate, the parts at the bottom left -- Indonesia, Australia
--
would move
up and to the right being parallel to the dashed line (the equator).